A wireless device that is able to communicate with others using radio frequency (RF) signals is usually equipped with an RF transmitter and receiver. An RF receiver employing a so-called superheterodyne architecture typically includes an antenna that transforms electromagnetic waves in the air into an RF electrical signal, a bandpass filter for separating a useful frequency band from unwanted frequencies in the signal, a low noise amplifier, a first mixer that translates a carrier frequency in the RF electrical signal into a lower and fixed frequency, which is an intermediate frequency (IF) equal to the difference between the carrier frequency and a local oscillator frequency, an IF filter, which is a bandpass filter centered on the IF frequency, and a second mixer that translates the IF signals to baseband so that the frequency spectrum of the resulting signal is centered on zero.
An RF receiver employing a homodyne architecture makes a direct conversion from the RF carrier frequency to the baseband usually with just one mixer, whose local oscillator is set to the same frequency as the carrier frequency in the received RF signal. With the homodyne architecture, there is no need for the IF filter, and only one mixer is required, resulting in lower power consumption and easier implementation of the receiver in an integrated circuit (IC) chip.
Some homodyne radios transceivers, such as interrogators or readers for radio frequency identification (RFID), are designed to receive a backscattered portion of a transmitted signal. RFID technologies are widely used for automatic identification. A basic RFID system includes an RFID tag or transponder carrying identification data and an RFID interrogator or reader that reads and/or writes the identification data. An RFID tag typically includes a microchip for data storage and processing, and a coupling element, such as an antenna coil, for communication. Tags may be classified as active or passive. Active tags have built-in power sources while passive tags are powered by radio waves received from the reader and thus cannot initiate any communications.
An RFID reader operates by writing data into the tags or interrogating tags for their data through a radio-frequency (RF) interface. During interrogation, the reader forms and transmits RF waves, which are used by tags to generate response data according to information stored therein. The reader also detects reflected or backscattered signals from the tags at the same frequency, or, in the case of a chirped interrogation waveform, at a slightly different frequency. With the homodyne architecture, the reader typically detects the reflected or backscattered signal by mixing this signal with a local oscillator signal.
In a conventional homodyne reader, such as the one described in U.S. Pat. No. 2,114,971, two separate decoupled antennas for transmission (TX) and reception (RX) are used, resulting in increased physical size and weight of the reader, and are thus not desirable. To overcome the problem, readers with a single antenna for both TX and RX functions are developed by employing a microwave circulator or directional coupler to separate the reflected signal from the transmitted signal, such as those described in U.S. Pat. No. 2,107,910. In another patent, U.S. Pat. No. 1,850,187, a tapped transmission line serves as both a phase shifter and directional coupler.
Because circulators are usually complex and expensive devices employing non-reciprocal magnetic materials, the use of a directional coupler is often preferred for low-cost radios. Conventional directional couplers, however, introduce losses in the receive chain. These losses may be tolerable for a radio transceiver operating in backscatter mode, where sensitivity is limited by spurious reflections of the transmitted signal from the antenna and nearby objects, but are objectionable when the radio is used as a pure receiver, as may be done for example in a LISTEN mode to detect nearby radios operating in the same band.